Lipid bilayer membranes are used to study various aspects of membrane structure and function. Electrical properties of bimolecular lipid membranes (BLMs) are measured to study: (1) mechanisms of opening and closing ion channels; (2) properties of ionophores isolated from natural cell membranes; (3) mechanisms of immune cytotoxicity. Diffusion and patching of fluorescent surface receptors on BLMs and on cell surfaces are studied by the technique of fluorescence photobleaching recovery: a small spot on the fluorescent-labeled surface is photolytically bleached by brief exposure to an intense laser beam, and subsequent recovery of the fluorescence is monitored. Fluidity of membrane is assessed by measuring fluorescence depolarization of membranes labeled with the fluorescent probe 1,6-diphenyl-1,3,5 hexatriene (DPH). The measurements are made to study the relationship between movement of cell surface receptors and cellular activation.